You’ll need 10 strands of lights—we used blue (50 TCP mini-LED Christmas lights; $16, Just Bulbs, 212-888-5707)—and a dark plastic or rubber kids’ ball about 12 inches in diameter. (A black basketball would work.) Wrap the first strand of lights around the ball as if you’re winding yarn, rotating as you go. Connect the plug of the first string to the second strand of lights and keep winding. As you wrap the rest of the lights, use the same method, doubling back when you near the end of a strand so that all the cord connections are close together and easy to hide. You can use twist ties to help hold the strands in place. This is a casual, playful display, so go ahead and snake the cord out of the fireplace to plug it in.